The present invention does in fact propose a novel glass which constitutes an improvement of the glass according to the U.S. Pat. No. 839,314 patent (Boudot et al.); it being possible for said novel glass to be tempered chemically in order to notably generate thin ophthalmic lenses (of about 1.5 mm thickness).
The various aspects of the invention--novel glass, novel strengthened glass, ophthalmic lenses--are described in detail further on in the present text.
The inorganic glass, as a constituent material of ophthalmic lenses obviously has the drawback of being much heavier than plastic. Consequently, the minimisation of the weight of ophthalmic lenses of inorganic glass has been sought for many years.
A first approach consists in reducing the density of said inorganic glass. Such a reduction is nevertheless limited by the necessity of having elements in the glass which enable obtaining the refractive index required.
Another approach, which is logical, consists in producing lenses of inorganic glass which are thinner, thinner lenses being also rendered more aesthetic. However, in order to satisfy the requirements of mechanical strength and resistance to impact of such thin lenses, it is necessary to strengthen said lenses. The strengthening by ionic exchange of such lenses has more particularly been considerably studied. During this strengthening by ionic exchange--carried out by chemical tempering--ions, usually alkaline ions, are exchanged on the surface of the glass. This enables increasing the constraint of compression on said surface and therefore the mechanical strength of the lens in said glass.
Such a method of strengthening has been carried out, with success, on conventional ophthalmic lenses which have a refractive index of 1.523 or 1.6. However, said conventional thin lenses, of said glass of relatively low index, does not enable obtaining satisfactory corrections.
The same method of strengthening has not led to satisfactory results with other inorganic glasses having a higher index (thus capable of enabling significant corrections), and especially with glasses having an index of about 1.7, as described in the U.S. Pat. No. 4,839,314 patent.
Said patent describes inorganic glasses which are particularly adapted to the production of ophthalmic lenses. These are glasses the composition of which, expressed in weight percentages based on the oxides, is given here:
SiO.sub.2 33-37 B.sub.2 O.sub.3 7.5-13 with SiO.sub.2 + B.sub.2 O.sub.3 44-48 Nb.sub.2 O.sub.5 8-10.5 ZrO.sub.2 4-6 TiO.sub.2 5-7 La.sub.2 O.sub.3 12.3-14.5 Li.sub.2 O 5-8 Na.sub.2 O 0-2.5 K.sub.2 O 0-2 with Li.sub.2 O + Na.sub.2 O + K.sub.2 O 5-8 CaO 8-9.5 SrO 2-4 As.sub.2 O.sub.3 0-0.8
Said glasses are characterised:
by a refractive index (n) of about 1.7 : this high index value is interesting in that it allows significant corrections under lesser thicknesses. The radius of curvature of the lens in said glasses may be increased; PA1 by an Abbe number (.sup..nu..sub.d) of at least 41: this high Abbe number value is also interesting. It indicates a low dispersion and therefore little chromatic aberration at the lens edge; PA1 a density of less than 3.25; PA1 a good acid durability.
The ophthalmic lenses produced from these glasses are therefore entirely satisfactory from the point of view of their optical properties. Their use does however remain limited insofar as, under low thicknesses, they cannot possess the mechanical properties required. As indicated above, it does in fact reveal to be impossible to strengthen the glasses in question by ionic exchange.
Thus, the ophthalmic lenses of said glass of high index must possess a minimal thickness of 2.0 mm in order to satisfy the ball-drop test in force in the United States, even though lenses of lower index which are strengthened by ionic exchange (see above) satisfy said test under thicknesses reduced to about 1.5 mm.
The Applicant, confronted with this technical problem of the conception of an inorganic glass having high index (of about 1.7), strengthable by ionic exchange (by chemical tempering), presently proposes a solution which constitutes the invention, it being entirely possible for said invention to be analysed as an improvement of the invention according to U.S. Pat. No. 4,839,314.
The Applicant presently proposes novel inorganic glasses which have compositions and properties which are around those of the glasses according to U.S. Pat. No. 839,314 but which surprisingly can be strengthened by ionic exchange (by chemical tempering).